Higher performance is being or will be required from many current or future electronic applications, especially vehicle-borne electronics intended for cars and ground-based means of transportation, aeronautical and medical systems or in-home automation solutions, for example. These applications for the most part require high-power switches (typically switching between 500 V and several kilovolts, with currents most often between 10 and 200 A) functioning in frequency ranges often above one megahertz.
Historically, high-frequency power switches have for a long time used field-effect transistors based on a semiconductor channel, most often made of silicon. At lower frequencies, junction transistors (thyristors, etc.) are preferred because they are able to withstand higher current densities. However, because of the relatively limited breakdown voltage of each of these transistors, power applications require many transistors to be connected in series, or wider transistors to be used, thereby resulting in a higher on-resistance. These series transistors generate substantial losses, both in the steady-state and switching regimes.
An alternative for power switches, especially high-frequency power switches, is the use of high-electron mobility transistors (HEMTs), also denoted by the term hetero-structure field-effect transistors (HFETs). Such transistors include a superposition of two semiconductor layers having different bandgaps and forming a quantum well at their interface. Electrons are confined to this quantum well and form a two-dimensional electron gas. For reasons of high-voltage and temperature withstand, these transistors are chosen to have a wide bandgap.
Among wide bandgap HEMTs, transistors based on gallium nitride are very promising. The width of their bandgap results in a higher avalanche voltage, compared to conventional electronic materials (Si, SiGe, GaAs, InP), in a high carrier saturation velocity, and in good thermal and chemical stability (enabling use in extreme environments). The breakdown field of gallium nitride (GaN) may thus be higher than 3×106 V/cm, thereby easily allowing transistors with breakdown voltages higher than 100 V to be produced (300 nm of GaN is sufficient). In addition, such transistors allow very high current densities to be obtained with lower resistive losses because of the very high electron mobility of the interface gas.
For certain applications, especially with a view to isolating a circuit in case of a malfunction of a control system, enhancement mode transistors are used, i.e. transistors with a positive switching threshold voltage, so that the transistor remains turned off in the absence of a control signal.
Because of the intrinsically conductive nature of the electron gas layer formed between a source and drain, it is technologically easier to produce a depletion mode heterojunction transistor. However, a number of fabrication processes have been developed with a view to forming enhancement mode heterojunction transistors.
According to a first approach, a layer of binary III-nitride is produced by epitaxy, then a layer of ternary III-nitride is produced by epitaxy to form an electron gas layer at the interface between these nitrides, then a p-type dopant such as Mg is implanted into the binary layer. Once the dopant implantation has been activated, the electric field generated by the implanted area allows an insulating zone to be created vertically above it, at the interface between the binary nitride layer and the ternary nitride layer. Thus, the conduction channel in the electron gas layer is depleted until a positive threshold voltage is achieved. However, it has been observed that such implantation creates defects in the structure (above all in the channel, the on-resistance of which is then increased), and that control of the implantation is imperfect, resulting in dopants being implanted in the channel (thereby further increasing its on-resistance).
According to a second approach, document WO 2005/070009 describes a fabrication process in which:                a first GaN layer is formed by epitaxy;        a p-type dopant implanted area is formed in the first GaN layer;        a second GaN layer is formed by epitaxy on the implanted area and the first GaN layer;        an AlGaN layer is formed by epitaxy on the second GaN layer; and        a gate is formed on the AlGaN layer plumb with the implanted area.        
However, the transistor thus obtained has drawbacks. Specifically, the strength of the field generated by the implanted area in the electron gas is poorly controlled. Hence, the threshold voltage of the transistor is also poorly controlled.